Prolonged physical effort in the heat frequently produces dehydration in athletes. To promote the completeness of rehydration prior to subsequent exercise, beverages containing sodium are beneficial. We explored the upper limit of sodium concentration given the need to keep the beverage palatable. PURPOSE: To determine the effect of sodium concentration ([Na+]) in a sports beverage on the completeness of rehydration that followed exercise-induced dehydration. METHODS: [Na+] included 0, 18, 30, and 60 mEq/L, the upper limit based on palatability issues. Following 24 h of standardized diet and activity, adult male endurance athletes (n=10) exercised in the heat for 90 min to induce 2.5-to-3.0 % reduction in body weight by sweating. No fluid consumption occurred during exercise. Thirty min after exercise, subjects were given 100% of the fluid needed to return their body weight to pre-exercise weight. Beverages were administered over 1 h in aliquots at 25%, 25%, 12.5%, 12.5%, 12.5%, and 12.5% of total sweat loss. Urine samples were obtained at standardized times over a 2-h period following completion of beverage ingestion. Nude body weights were measured before, during, and after the experiment. An ANOVA was used to compare cumulative urine excretion across the beverage treatments. RESULTS: Slight variations in sweat rate resulted in average fluid intakes of 1.8 to 1.95 liters (p>0.05). The results showed that cumulative urine excretion for 2 h was 0.546, 0.430, 0.322 and 0.287 liters for 0, 18, 30, 60 mEq/l beverages, respectively. Total percent of fluid retained relative to the volume ingested was 69%, 75%, 82%, and 83% for 0, 18, 30, and 60 mEq/L, respectively. Urine output for beverages 0 and 18 mEq/L were significantly (p<0.05) different from 30 and 60 mEq/L. CONCLUSIONS: Within the range of [Na+] that optimizes sensory acceptability, 30 mEq/L of sodium improved the completeness of rehydration following exercise-induced dehydration.